Traditional telephone service, commonly referred to as POTS (plain old telephone service), has been provided using analog signalling equipment. In an effort to meet the demand for high-bit-rate digital subscriber loop service (HDSL) that has accompanied the rapid growth of the digital computer industry, communication service providers currently use separately dedicated wire pairs, exclusive of those used for POTS service, for transporting digital signalling traffic at relatively high data rates (e.g, T1 signals at data rates of 1.544 Mb/s).
An example of such a high bit rate digital service network is diagrammatically illustrated in FIG. 1, wherein a 1.544 Mb/s signal (DS-1) 10 is coupled via a digital transceiver 12 to a local HDSL digital subscriber loop 14, which conveys the digital signals at the T1 data rate of 1.544 Mb/s, together with 16 kb/s of transport overhead to a network termination transceiver 16. From the network termination transceiver 16, respective digital signalling (DSL) channels are conveyed to customer premises digital terminal equipment. For the transmission of POTS signals, per se, FIG. 2 shows a conventional 64 kb/s PCM digital signalling link 20 interfaced via a CODEC 22 and SLIC 24 to a local analog loop 26, which serves a conventional analog telephone 28.
One proposal to integrate POTS service with high data rate digital service involves using frequency division multiplexing (FDM) of the digital data signals and analog POTS signals over the same wire-pair This technique, which is currently employed in asymmetric digital subscriber line (ADSL) systems and has been proposed for use in other DSL and HDSL systems, is shown diagrammatically in FIG. 3 as a combination of the digital only scheme of FIG. 1 with the POTS only scheme of FIG. 2.
Namely, at the office end of the local loop (which is typically powered from the central office), a 1.544 Mb/s digital signal (DS-1) 10 is interfaced via digital transceiver 12 to a first port 31 of an associated passband filter splitter/combiner 30, a second port 32 of which is coupled to the local subscriber loop 14. Similarly, the CODEC 22 and SLIC 24, which interface the 64 kb/s PCM digital POTS signalling link 20, are coupled over an analog line 25 to a third port 33 of filter splitter/combiner 30, so that, in the office-to-customer premises direction, as an example, the splitter/combiner 30 couples frequency division multiplexed HDSL signals and analog POTS signals over the same loop 14 to the customer site.
In a complementary fashion, the remote customer premises end of the local loop 14 is terminated at a first port 41 of a passband filter splitter/combiner 40 having a second port 42 coupled to HDSL transceiver 16, which interfaces respective DS-1 signals with locally powered customer premises digital terminal equipment 45. For the analog POTS signals, passband filter splitter/combiner 40 has a third port 43 coupled via local analog loop 26 to a conventional analog telephone 28.
A significant shortcoming of the FDM approach shown in FIG. 3 is the fact that, in order to obtain adequate filtering, the presence of the POTS signal removes 20-30 KHz of low frequency bandwidth that would otherwise be available for use by the digital (DSL) system, thereby reducing the effective range.